1. Field of the Invention
The invention relates to a means for supplying magnetically attractive developing powder, comprising a reservoir provided with an outflow opening and a magnetic system for supplying the magnetically attractive developing powder from the reservoir to a developing unit.
2. Description of Related Art
A supply means of this kind is known from U.S. Pat. No. 5,181,074, wherein a developing device is described for a developing powder made of toner and carrier particles. The carrier particles are magnetically attractive. In this device, the developing powder is transported to a supply unit.
The underside of the supply unit is closed by a magnetic roller. Rotation of the magnetic roller causes developing powder to be supplied to a mixing chamber of a developing unit, which is located in an electro-photographic printing unit. The mixing chamber is located beneath the supply unit.
In the practice of this device, it has been found that the quantity of developing powder that can be supplied is limited, even in the case of developing powder consisting solely of magnetically attractive toner. For example, when such so-called one component or unary toner is used, a supply speed of 30 to 35 grams per minute is the maximum obtainable. This quantity of toner is insufficient for copiers and printers operating at very high speeds (in excess of 100 copies or prints per minute).
Other methods of supplying large quantities of toner rapidly, for example using rotating mechanical valves, have the disadvantage that the supply toner comes into contact with the mechanical valves. This is a disadvantage when using toners which have been developed and are available today. These toners generally have relatively low fusing properties, i.e. the toner is fixed on the paper at a relatively low temperature. This is advantageous with regard to the energy required to fix the toner on the paper; however, these toners have a disadvantage in that they are mechanically vulnerable.
Lumping or caking of the toners occurs rapidly due to the mechanical load. This results in an unacceptable distortion of the image. The advantage of using a magnetic system such as a magnetic roller for supplying magnetically attractive toner or a toner-carrier mixture is that the toner is subjected to a low mechanical load. As a result, caking is avoided as much as possible.